1. Field of the Invention
The invention relates generally to chemical reactions conducted in the gas or vapour phase. Reactions may be exothermic or endothermic, and may be conducted over heterogeneous catalysts or under an appropriate form of external excitation.
Some particlar applications include ammonia synthesis, methanol synthesis, Fischer-Tropsch synthesis of hydrocarbons, steam reforming reactions, hydrogenation and dehydrogenation reactions, controlled oxidation reactions, and ozone generation.
2. Prior Art
Fundamental problems in chemical process industry include management of reaction equilibrium and kinetics to achieve high conversion with desired selectivity under moderate reaction conditions, and management of the heat of reaction to control reaction temperature and to achieve efficient energy recovery.
In the usual case that single pass conversion of the feed is incomplete because of equilibrium limitations, reaction equilibrium may be shifted (after selection of a favourable nominal reaction temperature and total pressure) by separation of useful products from the reactant/product mixture, and recycle of unconsumed reactants to the reactor. It is well known in the prior art to use any of the known separation processes based on condensation, distillation, membranes, absorbents or adsorbents downstream of a gas phase chemical reactor, to separate a product fraction from the reactor effluent, and return a recycle stream of reactants back to the reactor. In many important applications, a suitable separation process cannot operate at the temperature conditions of the reaction. Then the reactor and its associated product separation system are essentially incompatible, so that reactor effluent and recycle streams must be circulated through heat exchangers. The high cost of heat exchangers, recycle compressors, and other auxiliary equipment may then result in operation at relatively severe pressure or temperature conditions to minimize the need for recycle.
High temperatures generally promote good reaction rates, but shift the equilibrium of exothermic reactions toward lower conversion. Excessively high temperature is unfavourable for catalyst life, and may result in high pressure vessel costs. It is usually desirable to control reaction temperatures in a narrow range. With strongly exothermic or endothermic reactions, it is difficult to provide adequate heat exhange though reactor vessel walls or heat exchange surfaces. The common practice of quenching the reaction by "cold shot" injection of cool feed at intervals in the catalyst bed of exothermic reactions is highly wasteful of high grade heat of reaction. An alternative common practice of dividing an exothermic reactor into multiple staged reactors in series with intercoolers is capital intensive, and only roughly approximates isothermal conditions.
High capital costs are also associated with the well known tube reactor configuration for highly exothermic or endothermic reactions, in which multiple long narrow tubes in parallel are internally loaded with catalyst and externally immersed in a heat sink or heat source medium respectively. An exothermic tube reactor may function as a boiler, while an endothermic tube reactor may be a combustion heated furnace. Tube reactors have the performance limitations of large radial heat gradients and large axial pressure drops. Because of the limitations of heat exchange through reactor walls, fixed bed gas phase reactors are sometimes unable to exploit the full activity of the best available catalysts.
Heat transfer through reactor walls invariably results in substantial losses of thermal efficiency, particularly for highly exothermic or endothermic reactions with associated large heat fluxes. Such losses are unavoidably associated with use of a power cycle with a separate working fluid (such as steam) to recover exothermic reaction heat.
The above discussed general problems of managing both the reaction equilibrium and the heat of reaction are exemplified by the important process of ammonia synthesis. This exothermic reaction takes place over a promoted iron catalyst at a typical pressure of 200 atmospheres and a typical temperature of 750 degrees K. The hydrogen and nitrogen feed gases are stringently purified (apart from minor amounts of non-reactive "inerts" such as argon and methane), and are compressed to the high working pressure. In order to remove the product and thus maintain off-equilibrium conditions over the catalyst, the gas mixture of reactants, inerts and ammonia is circulated around a "synthesis loop" between the hot catalyst bed and a cool ammonia separator/condenser. This recirculation requires a recycle compressor and a large recuperative heat exchanger. To prevent excessive catalyst heating from the exothermic reaction, temperature control is achieved either by energy inefficient quenching by injection of cool feed gas, or by heat exchange to an external waste heat power recovery system which may drive feed gas or recycle compressors. A Brayton cycle gas turbine heat recovery approach for ammonia synthesis is disclosed by Barber et al in U.S. Pat. Nos. 4,224,299 and 4,273,743; while a supercritical Rankine cycle turbine for the same purpose is disclosed in U.S. Pat. No. 3,568,438 (Meienberg). Unless the synthesis loop operates at very high pressure, a refrigeration plant is needed to condense liquid ammonia at the cool end of the synthesis loop. Means are provided for purging accumulated inerts from the loop, and often to recover valuable hydrogen from the purge gas.
Considerable research attention has been devoted to improving productivity or selectivity of catalytic chemical reactors through cyclic operation forced by periodic variation of feed composition or reaction temperature. For example, it was found (A. K. Jain et al, "Forced Composition Cycling Experiments in a Fixed-Bed Ammonia Synthesis Reactor", ACS Symposium Series 196, pp. 97-107, American Chemical Society, 1982) that forced feed composition cycling at periods of several minutes improved the productivity of the ammonia synthesis reaction at relatively low working pressures. While it has been shown that in many cases cyclic operation can improve reaction productivity or selectivity under laboratory conditions, there remains a need for full scale reactors capable of beneficially exploiting a wide range of periodic phenomena which may be based on periodic forcing of feed composition, temperature, flow or pressure to achieve improved reaction performance relative to steady state operation.
Boreskov and Matros ("Unsteady-State Performance of Heterogeneous Catalytic Reactions", Catal.Rev.-Sci.Eng., 25(4), pp. 551-590 1983) described reactors for ammonia synthesis and sulphur dioxide oxidation, in which the flow direction through the catalyst bed is reversed at intervals to exploit the heat capacity of the catalyst bed as a thermal regenerator, preheating feed gas while providing enhanced conversion. Flow reversal is achieved by directional valves at both ends of the catalyst bed, operating on the high temperature feed and reactor effluent flows. In these reactors, the flow reversal means is entirely separate from any auxiliary apparatus that may be provided for energy recovery and product separation; and the reaction is performed at essentially constant pressure.
Chromatographic effects have been found to enable some reactions to be driven beyond normal equilibrium constraints. The reverse reaction can be suppressed by opposite separation of products, as pulses of a feed reactant migrate through a catalytically active adsorbent bed in the presence of a continously flowing carrier gas, which may be a second reactant. Chromatographic reactors are disclosed in U.S. Pat. No. 2,976,132 (Dinwiddie and Morgan) and in Canadian Pat. No. 631,882 (Magee). It was found by Unger and Rinker (Ind.Eng.Chem.Fundam. 15, p. 226 1976) that ammonia synthesis could be conducted to high conversion beyond usual equilibrium constraints at relatively low pressure, by pulsing nitrogen through a packed bed of ammonia catalyst mixed with molecular sieve adsorbent, with hydrogen as the carrier. These chromatographic reactors have severe limitations for practicable applications, including low catalyst productivity because of the time intervals between feed pulses, and the mixing of product components into a large excess of carrier gas which must be purified before recirculation. A related concept of conducting a chemical reaction within a parametric pump using catalytically active adsorbent was discussed by Apostolopoulos (Ind.Eng.Chem.Fundam. 14, p. 11, 1975), which would avoid the necessity for a carrier gas but would still have limitations of low catalyst productivity (because the desired separation is achieved only some of the time and is localized at any time to only part of the catalyst bed) and the lack of any provision for thermal energy conversion or recovery.
In principle, any thermodynamic power cycle may be coupled indirectly by heat exchangers to convert thermal energy associated with a chemical reaction. Internal combustion engines of course use a chemically reacting gas mixture as working fluid, but closed cycle engines such as the Stirling engine have not been applied to convert thermal energy associated with a chemical reaction of their working fluid, by definition of the closed cycle and also their practical limitations. It is noted that use of rapidly dissociating gases such as nitrogen tetroxide as working fluid in Brayton (gas turbine) and Stirling closed cycle engines is proposed in U.S. Pat. No. 3,370,420 (Johnson). The chosen gas dissociates to increase volume when at high temperature, and recombines to reduce volume when at lower temperature. This effect is intended to improve the cycle "work ratio", the ratio between work of expansion at high temperature and work of compression at low temperature. Use of a catalyst in the regenerator of a closed cycle Stirling engine to speed the dissociation and recombination reactions has been proposed in U.S. Pat. No. 3,871,179 (Bland), again with the object of obtaining high work ratio. In these inventions, the forward reaction is exactly cancelled by the reverse reaction over each cycle, because the gases are trapped in the engine working spaces and there are no means to drive the net reaction off equilibrium or deliver products. The dissociation and recombination reactions take place in the Stirling engine regenerator, where their heat of reaction is cyclically stored and returned to the working fluid. This heat of reaction cannot participate directly in Stirling cycle energy conversion between heat and mechanical work, as heat converted and transported by the Stirling cycle must be manifested as heat of compression or of expansion in the variable volume working spaces or cylinders of the Stirling machine.
As the above-cited Johnson and Bland inventions have no means to separate and remove any product from the reversible reaction of the working fluid, they cannot be applied to chemical synthesis processes. Therefore it has not hitherto been possible to apply the closed Stirling cycle to recover heat from exothermic chemical synthesis reactions, or to supply heat to endothermic reactions, while using the reacting gases as Stirling machine working fluid.
Pressure swing adsorption is a well known gas separation process, previously applied to purifying the feed gas to a chemical reactor and to separating the product/reactant mixture effluent from the reactor as disclosed for example by Lassmann et al (U.S. Pat. No. 4,280,824).
Further prior art relating to pressure swing adsorption applied to gas separations is outlined in my co-pending patent application Ser. No. 06/866,395. None of those references contemplates the direct coupling of a pressure swing adsorption process with a regenerative thermal power or heat pump cycle as in the present invention.